Update seastar submodule
Update seastar submodule

* seastar 85ca12d...31b925d (19):
  > net/byteorder: fix 64 bit ntohq and htonq on big endian machines
  > core, util: fix compilation on non-x86 processors
  > core/memory: Fix SIGSEGV in small_pool::add_more_objects()
  > log: remove debug leftovers
  > Merge "TLS state machine fixes" from Calle
  > logger: allow adjusting the timestamp style for stdout logs
  > thread: make thread_context::s_main portable
  > core: add seastar::cache_line_size constant
  > Add detach() to input_stream and output_stream
  > Install dependencies for Arch Linux.
  > tls: Guard non-established sockets in sesrefs + more explicit close + states
  > tls: Make vec_push fully exception safe
  > basic_sstring: resize uses sstring
  > Merge "Add and correct unit tests" from Jesse
  > tcp: enforce 1-byte maximum segment invariant with zero window
  > tcp: verify 1-byte maximum segment invariant during send with zero window
  > memory: reduce small_pool vulnerability to fragmentation further
  > Prometheus: avoid merging all metrics family
  > net: Fix possible NULL pointer dereference.

duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,scylladb/scylla